The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Mediator for Tasmanian bus deadlock

A mediator has been called in to try to resolve an increasingly bitter pay dispute between Tasmanian bus drivers and the state government.

Drivers marched on Parliament House in Hobart on Wednesday, angry they had been locked out of work with all services cancelled by state-owned transport company, Metro.

Around 100 gathered outside Metro's headquarters and 60 took part in the March after two planned stop-work meetings led to the shutdown.

The state's 400 drivers are seeking an annual pay rise of three per cent over three years, while Metro has offered 2.1 per cent and a faster progression to the top rate.

Services have been disrupted since action began more than a month ago and Metro chief executive Heather Haselgrove said an independent facilitator would meet both parties next week in an attempt to end the deadlock.

"Metro has always said that our door is open to continue negotiations so we are pleased with this outcome," Ms Haselgrove said in a statement.

Commuters including school students returning after their Easter break were forced to make other transport arrangements on Wednesday.

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Tasmania's Minister for Sustainable Transport Nick McKim has been criticised by the Rail Tram and Bus Union and the state's Liberal opposition for failing to intervene in the dispute.

The opposition says the dispute has exposed a split in the power-sharing arrangement between Mr McKim's Greens and Labor cabinet members who opposed stand downs.

"If this government can't even agree on how to run the buses, how can we trust them to agree on how to run the State?" opposition spokesman Matthew Groom said.

Mr McKim said the dispute needed to be resolved by the parties.

"This is a matter ultimately between Metro and its workforce and I've continually urged both parties to sit down at the negotiating table and work their way through this issue, and I still believe that's what needs to happen," he told ABC radio.